1. Field of the Invention
The present invention relates to an image encoding/decoding technology including an intra-prediction.
2. Description of the Related Art
In MPEG-4 AVC/H.264 (hereinafter, referred to as “AVC”), improvement of encoding efficiency in a frame has been enhanced by employing an intra-prediction wherein a processed pixel in the same frame is referenced. In AVC, after an encoding target frame is divided into a macroblock (16×16 pixel unit), the macroblock is further divided into a subblock such as an 8×8 pixel unit or a 4×4 pixel unit, and is processed.
FIG. 1 is a diagram showing a processing order of encoding of a macroblock. A reference sign 101 denotes the processing order of the macroblock, a reference sign 102 denotes the processing order of a 4×4 subblock, and a reference sign 103 denotes the processing order of an 8×8 subblock, the respective processing orders being shown by the arrows. Intra-prediction processing is, at each time, performed using decoded pixels of a subblock positioned at an upper side and a left side, the processing of which has been already completed.
FIG. 2 is a diagram illustrating intra-prediction processing. In an intra-prediction of the 4×4 subblock, 9 modes denoted by a reference sign 201 in FIG. 2 are defined in advance, and one of the 9 modes is selected with respect to each subblock. One of the 9 modes is a DC mode. A prediction signal uses a uniform value with respect to a prediction direction shown by the arrow.
An example of the prediction signal is shown by a reference sign 202 in FIG. 2. Reference signs 203 to 206 denote decoded subblocks, and a reference sign 207 denotes a target subblock to be processed. When a horizontal direction mode is selected as a prediction mode of the reference sign 207, the prediction signals (a to p) are a=b=c=d=I, e=f=g=h=J, i=j=k=l=K, m=n=o=p=L, and when a vertical direction mode is selected, the prediction signals (a to p) are a=e=i=m=A, b=f=j=n=B, c=g=k=o=C, and d=h=l=p=D.
Japanese Patent Application Laid-Open No. 2009-284275 discloses a technology that improves prediction accuracy by taking an arbitrary prediction direction.